您不能使用浮点整数等值，例如文本字段：

String query = "UPDATE Table_com SET Contents='" + textBox6.Text + "' WHERE Variable='" + comboBox5.Text + "'";

并且您的代码确实存在风险，您可以使用像这样的参数，不要使用字符串，并将值转换为float：

static void Main(string[] args)
        {
            var ConnectionString = "YOUR CONNECTION STRING";
            DataTable dt = new DataTable();
            using (SqlConnection con = new SqlConnection(ConnectionString))
            {
                String query = "UPDATE Table_com SET Contents=@contents WHERE Variable=@variable";

                using (SqlCommand cmd = new SqlCommand(query,con))
                {
                    cmd.CommandType = System.Data.CommandType.Text;

                    cmd.Parameters.Add("@contents",SqlDbType.Float).Value = Convert.ToDouble(textBox6.Text);
                    cmd.Parameters.Add("@variable",SqlDbType.NVarChar).Value = comboBox5.Text;

                    using (SqlDataAdapter da = new SqlDataAdapter(cmd))
                    {
                        da.Fill(dt);
                    }
                }
            }

        }

这是我尝试引用Wiki可以回答的问题

 SqlConnection con = new SqlConnection(@"Data Source=DESKTOP-VEFPLGG\SQLEXPRESS;Initial Catalog=test;Integrated Security=True");    
 private void comboBox5_SelectedIndexChanged(object sender,EventArgs e)
    {

        DataTable dt = new DataTable();
        String query = "UPDATE Table_com SET Contents=@contents ";

            using (SqlCommand cmd = new SqlCommand(query,con))
            {
                cmd.CommandType = System.Data.CommandType.Text;

                cmd.Parameters.Add("@contents",SqlDbType.Float).Value = textBox6.Text;
                //cmd.Parameters.Add("@variable",SqlDbType.Float).Value = Convert.ToDouble(comboBox5.Text);

                using (SqlDataAdapter da = new SqlDataAdapter(cmd))
                {
                    da.Fill(dt);
                }
            }
    }